Several general developmental problems using potential oral-facial tissues are under study: The development of organized structural connective tissues of the mandibular arch of the embryo are under study using in vitro procedures. Particular differentiation of bone and cartilage. The capabilities for differentiation of arch tissues are studied by growth of the siolated tissues from embryos of different age. These studies seek to determine the earliest embryonic age at which mandibular arch tissues are able to differentiate independently of other tissues. Heterologous tissue recombinations are planned with the goal of understanding the developmental interrelationships between the arch tissues. Scanning electron microscopic studies are utilized to observe the surfaces of the cells and tissues prior to and following their differentiation and to correlate the appearance of the cells with the known developmental dissimilarities between the different portions of the arch.